In MIMO (multiple input multiple output) OFDM (orthogonal frequency division multiplexing) systems, there are multiple transmit antennas and multiple receive antennas and a plurality of sub-carriers that are available for transmission between the transmit:antennas and the receive antennas for either one or multiple users. New advances in MIMO OFDM systems are taught in Applicant's co-pending application Ser. No. 10/593,053 entitled “Pilot Design For OFDM systems With Four Transmit Antennas” filed Mar. 15, 2005, and in Applicant's co-pending application Ser. No. 11/547,077 entitled “Wireless Communication Methods, Systems, And Signal Structures” filed Apr. 4, 2005, both hereby incorporated by: reference in their entirety. With open loop implementations, the transmitter transmits on the multiple transmitter antennas and sub-carriers without the benefit of channel information fed back from the receivers.
Efforts have been made to facilitate wireless closed-loop MIMO communications including broadband closed-loop MIMO, which might for example be based on OFDM modulation schemes, and narrowband closed-loop MIMO. Broadband closed-loop MIMO includes many sub-bands. Each of these sub-bands requires MIMO channel feedback for a closed-loop implementation. As a result the feedback resources required for broadband closed-loop MIMO can become quite large. Narrowband closed-loop MIMO, by comparison, includes one or a few sub-bands and requires a relatively smaller amount of feedback resources. Broadband and narrowband MIMO, therefore, have different applications.